This invention relates to a novel and unique method of generating hydrogen from water. More particularly, this invention pertains to a novel method of producing hydrogen from water using metal-ceramic composites, at ambient or elevated temperature and at neutral, or close to neutral, pH.
The generation of hydrogen utilizing inexpensive simple processes is becoming increasingly important. The increasing demand for hydrogen arises from the imminent paradigm shift to a hydrogen-based energy economy, such as in hydrogen fuel cells. This shift approaches as the worldwide need for more electricity increases, greenhouse gas emission controls tighten, and fossil fuel reserves wane. The attendant market for fuel generators addresses the near term lack of hydrogen supply infrastructure that is necessary for the proliferation of the hydrogen fuel cell. Hydrogen-based economy is the only long-term, environmentally benign alternative for sustainable growth. Over the last few years it is becoming more apparent that the emphasis on cleaner fuel will lead to use of hydrogen in a significant way. Providing that renewable energy sources, such as hydroelectricity or solar energy, are used to produce hydrogen through decomposition of water, there are no environmental threats produced by the hydrogen economy.
The common method to recover hydrogen from water is to pass electric current through water and thus to reverse the oxygen-hydrogen reaction, i.e. in water electrolysis. Another method involves extraction of hydrogen from fossil fuels, for example from natural gas or methanol. This method is complex and always results in residues, such as carbon dioxide, at best. And there is only so much fossil fuel available. In these reforming methods the resulting hydrogen must be somehow stored and delivered to the user, unless the hydrogen generation is performed xe2x80x9con-boardxe2x80x9d, close to the consumption system. The safe, reliable, low-cost hydrogen storage and delivery is currently one of the bottlenecks of the hydrogen-based economy. The current invention addresses this problem through safe, xe2x80x9con-board/on-demandxe2x80x9d production of hydrogen close to the user systems, using simple, safe and pollution-free metal-ceramic composites reacting with water.
This invention relates to a novel method of generating hydrogen from water. Water consists of two elements: oxygen and hydrogen. A relatively large amount of energy is released when these two elements react to form water. This energy may be captured and efficiently converted to electricity in fuel cells. More importantly, nothing else is released when oxygen and hydrogen react to form water.
Consequently, the hydrogen-oxygen reaction is potentially a pollution-free source of energy. Although about 20% of air is oxygen, there is no easily accessible, safe source of hydrogen available. The current invention addresses this problem.
There are only a few resources that can produce abundant hydrogen and these include hydrocarbons and water. Of these, the only pollution free source of hydrogen is water. One of the problems that must be addressed before the new hydrogen economy replaces the current xe2x80x9coil/gas/coal/nuclearxe2x80x9d economy, is finding a safe, environmentally benign and cost-effective method of generation, storage and distribution of hydrogen. This issue is the primary focus of the present invention.
It is known that some metals produce spontaneously hydrogen in contact with water. These are, for example, alkaline metals such as potassium (K) or sodium (Na). These metals could be used as water-split agents through the simple reaction which proceeds spontaneously once metal is dropped into water:
2K+2H2Oxe2x86x922KOH+H2xe2x80x83xe2x80x83(1)
Similar reactions can be written for other alkalis, e.g. Na. Unfortunately the residual hydroxide product (i.e. KOH in the above reaction) causes very high alkalinity of the resulting products, making them corrosive, dangerous to handle and potentially polluting to the environment. As the reaction (1) proceeds spontaneously and violently, the reacting metals must be always protected from undesirable contact with water (i.e. effectively also from air which under normal conditions will contain water vapor). This increases costs of the technology and adds safety and pollution problems. The reaction products are not easy to handle and recycle. Reaction (1) has an advantage in that the reaction products (i.e. KOH) continuously dissolve in the reacting water, and thus allow the reaction to continue until all metal reacts. Similar effect was difficult to achieve with other attractive metals such as aluminum, as in this case the reaction products, i.e. Al(OH)3, tend to deposit on the surface of the reacting metal and thus restrict access of reactants (e.g. water or oxygen) to metal surface, eventually stopping the reaction. This xe2x80x9cpassivationxe2x80x9d phenomenon is a fortunate property of reactive metals such as Al, as it preserves them in substantially corrosion-free state in wide variety of applications, as long as environment is not too acidic or alkaline. At the same time, passivation does not allow to use Al for generating hydrogen from water at close to neutral pH. The presently disclosed invention teaches a simple method preventing formation of the passivation layer of products on the Al surface, and thus allows to use Al for generation of hydrogen from water at close to neutral pH.
The research intensity, and the proportional literature volume pertaining novel means of hydrogen generation and use, is extremely large and increasing in recent years. Below we present the selected patent publications that may have some relationship to the present invention. A number of variants of water split reaction to produce hydrogen have been disclosed in the past, primarily involving alkali metals. Two patents (U.S. Pat. No. 5,817,157 and 5,728,464) that describe a system for the controlled generation of hydrogen from spherical polyethylene-coated Na or NaH pellets have been issued to Jed Checketts [1,2]. The system comprises a container to hold the pellets and water, a hydraulic system for splitting open the pellets, and a hydrogen sensor and computer which provides a feedback loop for activating the pellet splitter. This technology supercedes other patents that have been issued for controlled hydrogen generators that employ alkali metals (U.S. Pat. No. 4,356,163 [3]; 5,514,353 [4]; 3,716,416 [5]) or metal hydrides (U.S. Pat. No. 5,593,640 [6]) or iron (U.S. Pat. No. 5,510,201 [7]) and water. Another patent describes a generator that employs hydrochloric acid and pure metal (U.S. Pat. No. 4,988,486 [8]). Additional patents have been issued for the generation of hydrogen gas in an uncontrolled manner (U.S. Pat. No. 5,143,047 [9]; 5,494,538 [10]; 4,600,661 [11]; 4,072,514 [12]; 4,064,226 [13]; 3,985,865 [14]; and 3,966,895 [15]) in systems comprising mixtures of alkali or alkali earth metals and/or aluminum and water or aqueous salt solutions. Although in one particular case an Al-ceramic composite has been proposed to initiate the water split reaction (JP. Pat. No. 1,061,301 [16]), the ceramic is calcined dolomite, i.e. calcium/magnesium oxide. Once contacted with water, these oxides cause very substantial increase of pH, which stimulates corrosion of Al with accompanying release of hydrogen. The system has all the disadvantages of water split reaction using alkaline metals, i.e. very high alkalinity and difficult recyclability of the products. In one case, the Mg and Al are mechanically ground together to form a composite material which is then exposed to water (U.S. Pat. No. 4,072,514 [17]). Continuous removal of the passivation layer on aluminum by mechanical means, in order to sustain aluminum assisted water split reaction, has been disclosed in (FR Pat. No. 2,465,683)[18]. This patent describes a method of automatic gas production by reaction of alkaline solution with metal-incorporating feeding without interruption of reaction and continuous metal cleaning applicable in producing hydrogen for energy source. For hydrogen production aluminum on sodium hydroxide solution in water was used.
A composite material comprising a mechanical mixture of aluminum oxide(s) and/or aluminum hydroxide(s) and aluminum (Al) metal, which when submerged in water, produces hydrogen gas at or near to neutral pH. The phenomenon has been demonstrated reproducibly. The evolution of hydrogen gas is dependent on several factors, namely, temperature, pH, proportion and particle size of ingredients and mixing conditions. The essential water split reaction is as follows:
2Al+6H2Oxe2x86x922Al(OH)3+3H2{9 greater than pH greater than 4}xe2x80x83xe2x80x83(2)
The key feature of the present invention is that the reactant system is able to sustain the aluminum-assisted water split reaction, equation (2), but in neutral, or close to neutral conditions, e.g. in tap water.
If tap water is used (as in plurality of experiments described below) the only products of reaction (2) (i.e. after completion of the reaction) are aluminum oxide(s), aluminum hydroxide(s) and hydrogen. Aluminum oxide and hydroxide are readily recyclable back to aluminum metal through the well-known electrolysis process. The hydrogen, thus generated, can be subsequently oxidized to water in the fuel cell. The resulting water can be feed back to sustain the water split reaction (2). The life-cycle loop for hydrogen generation through aluminum assisted water split is thus closed with no impact on the environment, especially if electrolysis of alumina (to produce Al) is performed using hydroelectric or other renewable form of energy.
The principal discovery disclosed in the present invention is that the reaction (2) is sustained, i.e. passivation layer of reaction products does not hinder the reaction, if the reacting aluminum metal is in contact with externally supplied nonmetal (ceramic) such as aluminum oxide(s) or hydroxide(s). Thus, a composite material comprising mechanical mixture of aluminum metal (Al) and aluminum oxide(s) or hydroxide(s), when submerged in water, continuously produces hydrogen gas. The reaction proceeds for the mass ratio of AM to the oxide(s) or hydroxide(s) varying over the whole range, from a few percent to up to 99% of the additive(s). Similarly, the reaction proceeds in a wide range of acidity/alkalinity (pH) of water, e.g. 11 greater than pH greater than 2, and water temperature, e.g. from about 10xc2x0 C. to 90xc2x0 C. Although the reaction proceeds faster at elevated temperatures, water acidity/alkalinity in the range 9 greater than pH greater than 4 has relatively weak effect on the reaction rate. The phenomenon has been demonstrated reproducibly, as illustrated in the following figures and examples.
The system disclosed in the present invention may accelerate introduction of hydrogen-derived power to consumer electronics (e.g. laptop computers) or transportation. For example, according to reaction (2) the aluminum assisted water split leads to generation of about 1.2 cubic meters of hydrogen (at standard conditions) out of 1 kg of aluminum reacting with water. This is about 30% more than the amount of hydrogen produced through rather complex process of reforming 1 kg of methanol, which is one of the methods proposed for supplying hydrogen to fuel cells. More importantly, there is no carbon dioxide/ monoxide produced in aluminum assisted water split reaction. This is especially important for application in fuel cells, where small amount of CO contaminant in hydrogen may poison the additive and make the cell dysfunctional. The xe2x80x9cstorage ratioxe2x80x9d, i.e. the mass ratio of the hydrogen generated to the metal reactant, is therefore about 11%, substantially more than any other currently known means of on-board hydrogen storage, e.g. through metal hydrides (the mass of water is neglected in the storage ratio as it may be partially re-circulated within the system, or replenished through abundant distribution system in place). As aluminum, aluminum oxide and aluminum hydroxide are the safest materials known to humanity (e.g. are commonly used in food, drug, cosmetics etc. products), the novel process promises to be safe and manageable by simple means.